If my opponent has a Robo Substitute on the bench, does that add 20 damage to Exeggutor's "Shake It Off" attack?
Yes, Robo Substitute counts as a {C} Pokemon while in play.
Source: Roaring Skies FAQ (2015-05-07), TPCi Rules Team (2015-05-07)

Relevant cards

Note: All applicable cards may not be listed below.
XY4_102
XY6_2